The invention relates to a GIS (Gas Insulated (Metal-enclosed) Switchgear) foundation bolt looseness diagnosis method based on frequency response curve peak distribution. The method is characterized in comprising the following steps that: 1) selecting one point on the center of the top surface of a GIS tank body as a vibration measurement point P, and meanwhile, selecting one point on the center of the side surface of the same GIS tank body to serve as an excitation point E; 2) utilizing an impact force hammer to apply pulsed excitation x(t) on the excitation point E, and picking up a vibration signal y(t) from the vibration measurement point P through an acceleration sensor; and 3) utilizing the H1 method of a frequency response function to estimate GIS frequency response characteristics.Compared with traditional loop resistance testing and winter analysis, the diagnosis method for a GIS mechanical state has the advantages that live detection can be carried out and the diagnosis method is tightly related to GIS basic machine filling, the inherent mechanical state of the GIS can be effectively represented, the change rule of the inherent mechanical characteristics of the GIS before and after faults can be obtained, and therefore, the live detection of the GIS mechanical state is realized.